1. Field of the Invention
The present invention relates to an image reading apparatus for a slide film or the like.
2. Related Background Art
In a prior art slide film image reading apparatus, a 35 mm light permeable film 30 to be read is mounted on a slide mount 32 during the read operation, as shown in FIGS. 1 and 2. In this case, in a conventional image reading apparatus, the slide mount 32 is held by a film retention spring 36 of a slide mount table 34.
However, a thickness of the slide mount may differ from type to type of the slide film and an optical axial position of the slide film may significantly deviate from a design optimum point of the apparatus. Thus, exact focusing may not be attained depending on the type of the slide film and a resolution of the read image data may be lowered.
In order to solve the above problem, a method for inserting a spacer 38 of an appropriate thickness between the film mount 32 and the film mount table 34 has been proposed, as shown in FIG. 3. In this method, however, whenever a different type of slide film is to be used, one of several spacers having different thickness prepared by a manufacturer, which is optimum to the film mount to be used should be selected. This is troublesome work. Further, if an improper spacer is selected, it is not detected until the read data is outputted to a CRT display after a series of image read operation and the result is visually checked. Accordingly, whether the apparatus fully performs its function or not largely depends on the experience and judgement of the user.
A method for moving the slide film along the optical axis has been proposed. However, this method requires not only a sub-scan direction drive mechanism for reading the image but also an optical axis drive mechanism for focusing, and the apparatus is complex.
A technique to focus by moving a projection lens itself is common in a focusing device but magnification changes each time a different type of slide film is used.
Apart from the above problems, the apparatus has another problem as described below.
The slide film to be read by the apparatus usually has a strong warp property. Thus, because of a mount operation of the film to the film mount or subsequent temperature and humidity change, the film has more or less warp even after it has been held by the film mount.
In a reading apparatus having a fixed relative positional relationship between the slide film and the optical axis of the focusing lens and the image pickup element, focusing may be attained within the read range even if a certain degree of warp is included, if a depth of focus of the optical system is large.
This method is explained with reference to FIG. 4. Numeral 15 denotes a film, numeral 16 denotes 10 a section of a film mount and numeral 18 denotes a depth of focus.
It shows that the film 15 includes a warp relative to a direction 17 of film movement during the image scan but the entire film 15 is within the depth of focus 18. In this state, the film 15 is focused everywhere.
However, if the film 15 includes a warp which is beyond the depth of focus, defocusing may occur depending on a read area on the film 15. FIG. 5 shows such a state. Since the warp of the film 15 is larger than the depth of focus 18, defocusing occurs at an area 19 of the film 15.
Increasing the depth of focus means the darkening of the optical system and it requires expanding a storage time so that the image pickup element can acquire a sufficient amount of light. As a result, an S/N ratio of the image signal is lowered.